OS X Mountain Lion: We have access to all types of ways to remind ourselves to do things—whether it's to-do lists, task lists, or calendars. But if you're looking for something much simpler that just reminds you to do something in a few minutes, or an hour, Scheduled Notifications does the job.

Scheduled Notifications sits in your menu bar and works with Notification Center as a lightweight system to give yourself quick reminders. It's not meant as a to-do list. Scheduled Notifications is meant more as a disappearing sticky note that reappears when the time is right. You can set Scheduled Notifications to notify you at a specific time, or just type in something like, "in 20 minutes."